The present disclosure relates to alkali halide scintillation crystals, and more particularly to methods by which alkali halide scintillation crystals may be manufactured. In embodiments, the present disclosure relates to hot forging of Nal(TI) scintillation crystals to produce scintillators having a large area, a rectangular shape, and uniform properties.
Manufacturers of alkali halide crystals (including Nal(TI) crystals) have long attempted to enlarge and shape melt-grown crystal ingots by forging them at elevated temperatures. Crystal forging techniques are taught, for example, in U.S. Pat. Nos. 3,933,970 and 4,171,400. These patents disclose hot forging in which there is a gradual compression of the crystal ingot between two surfaces. If a cylindrical ingot is flattened parallel to its axis, the forging process produces a disk-shaped crystal having a circular cross-section; if the cylindrical ingot is flattened perpendicular to its axis, the forging process produces a parallelepiped-shaped crystal having a rectangular cross-section. One problem with these processes is they may produce forged crystals having cracks and fissures at and adjacent the periphery of the crystal. As a result, the peripheral region of the forged crystal is not useful and must be cut away. This limits the size of the crystals that can be produced using the forging process. It also reduces the yield of the process because only a fraction of the forged crystal is useful with the rest being discarded or recycled.
U.S. Pat. No. 5,792,253, the entire disclosure of which is incorporated by reference herein, discloses a method of forging a cylindrical ingot of an alkali halide. A melt-grown ingot is flatted parallel both to its axis and to a particular crystallographic plane; the particular plane is determined by the crystal structure of the alkali halide to be forged. Then, the flat is placed on the lower platen of a heated dual-platen press and the ingot is compressed (advantageously, by raising the lower platen) between the platens while in a plastic state. The method produces a forged ingot in which cracks and fissures are absent from the periphery of the ingot.
Similarly, U.S. Pat. No. 6,224,666, the entire disclosure of which is incorporated by reference herein, discloses methods for forging cylindrical ingots of an alkali halide. In this case, the melt-grown alkali halide ingot is still heated to plasticity and compressed between the heated platens of a dual-platen press but, before the compression begins, the ingot is placed mid-way between two parallel, planar, and vertically extending barriers. The ingot is positioned so that its axis is vertical and so that the barriers are maintained in a predetermined relationship with the crystal lattice structure of the ingot. Then, the ingot is compressed along its axis to form a block.
The above-processes may avoid problems with cracks and fissures forming at and adjacent the periphery of the crystal.
In some instances, the prior art methods include the application of layers of fiberglass cloth impregnated with boron nitride on the platens to prevent the cloth from sticking to the ingot. One problem with this approach is fibers from the fiberglass cloth may become embedded in the ingot, thereby forming impurities in the resulting crystal and rectangular plates.
It would be desirable to develop methods for forming rectangular scintillators having a large area, rectangular shape, and uniform properties with reduced impurities.